cmd/compile: mark type descriptors as always dupok

The types of the two interfaces should be equal, but they aren't.
We end up with multiple descriptors for a type when we need type
descriptors to be unique.

Fixes #49241

Change-Id: I8a6c70da541c6088a92a01392bc83b61cc130eba
Reviewed-on: https://go-review.googlesource.com/c/go/+/360134


Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>